Heat exchangers, such as steam generators and, in particular, the steam generators of pressurized-water nuclear reactors, generally comprise a bundle of tubes of great length and small diameter forming the exchange surface and permitting the heating and the vaporization of the feed water of the steam generator.
In an electronuclear power station whose reactor is cooled and moderated by pressurized water, the heat released by the nuclear reaction is removed from the core by the cooling fluid or primary fluid and is transferred in the steam generator to secondary water which, after vaporization, drives the turbo-generator sets of the power station. This secondary water is returned in liquid form into the steam generator, after passing through the condenser.
The exchange surface of a steam generator of a pressurized-water nuclear reactor consists of a large number of tubes (for example, 3,400 tubes for each one of the three steam generators of a 900 MW.e power station), inside which the primary fluid circulates. The secondary fluid comes into contact with the outer surface of the tubes.
The tubes have an internal diameter of approximately 20 mm and are fixed at each of their ends into bores passing through a tube plate of great thickness, this thickness being of the order of 550 mm.
The joint between the tube and the tube plate is provided by expansion of the tube in a corresponding bore passing through the plate and by a weld made at its lower end.
The expansion of the tube may be achieved substantially over the entire length of the bore passing through or, on the other hand, over only a part of this length.
The tubes of the bundle of a steam generator form not only the heat exchange surface between the primary fluid and the secondary fluid, but also a confinement wall for the primary fluid, fulfilling an extremely important function in respect of the operating safety of the nuclear installation.
In the case of a power station comprising a pressurized-water reactor of 900 MW.e power, the primary fluid is at a pressure in the region of 155 bars and at a temperature of 300.degree. C. and the secondary fluid is at a pressure of 56 bars and at a temperature of 271.degree. C.
The difference in pressure existing between the primary fluid and the secondary fluid results in a situation whereby deterioration of a tube of the bundle of the generator can lead to a leakage of primary fluid into the secondary fluid. The primary fluid is charged with radioactive substances in solution or in suspension and, consequently, even a small amount of leakage in a tube of the bundle of the steam generator leads to contamination of the secondary water and of the components of the power station in which this secondary water circulates. A defective operation regime of this type is unacceptable since the secondary fluid circulates outside the containment buildings of the nuclear reactor in the turbine set and in all the auxiliary circuits and apparatuses which are associated with this set.
The tubes of the bundle of a steam generator are designed and dimensioned so that they can be subjected, without damage, to the various mechanical and thermal loads which they undergo in service; the material from which they are made is defined in order to avoid, as far as possible, corrosion of these tubes by the fluids with which they come into contact.
Moreover, the chemical characteristics of the primary and secondary fluids are, during operation of the installation, continuously monitored and, if appropriate, rectified, in order to reduce corrosion risks.
However, it is necessary to continuously ensure that the tube bundle of the steam generator is in a satisfactory condition and completely separates the primary and secondary fluids. This monitoring is performed using continuous surveillance, during operation, of the level of activity in the secondary water, which makes it possible to detect leakages whose flow rate is very small. During periods of shutdown of the nuclear installation, the tubes of the bundle are examined, for example using eddy currents, in order to detect defects whose progression could subsequently lead to leakage.
Despite the various precautions taken both at the design and manufacturing stage and during operation of the steam generators, it became apparent that some materials used for manufacturing the tubes of the bundle were quite sensitive to stress corrosion. This applies particularly to some types of nickel-based alloys containing chromium and iron.
Stress corrosion principally develops in the zones where the tube is subject to residual stresses and, in these zones, a crack may form across the thickness of the tube, which is liable to result finally in leakage of primary fluid into the secondary fluid.
A zone which is particularly sensitive to this type of corrosion, in the case of a tube crimped along the entire length of a bore passing through the tube plate, is located at the level of the upper face of the tube plate. In fact, after being inserted into the tube plate and before its lower end is welded, the tube is subjected to an operation of crimping by diametrical expansion, known as widening or expansion by rolling, and which aims to ensure intimate contact between the outer surface of the tube and the surface of the bore pierced in the tube plate. Widening of the tube may take place over the entire height of the tube plate in order to eliminate the gap resulting from the diametrical play between the tube and the bore in the plate, this gap forming a semiconfined space in which concentrations of secondary water may occur, leading to considerable corrosion phenomena.
Crimping of the tube may also be performed over only part of the length of the bore passing through the tube plate, this partial crimping generally being performed in the vicinity of the end of the bore located towards the entry face of the tube plate.
When the tube is crimped, there remains in the wall of the tube a zone of transition between the part of the tube which is widened and in contact with the bore of the tube plate and the upper part of the tube which has not been subjected to diametrical expansion. In this transition zone, the tube is subject to residual stresses which, if the material is sensitive to stress corrosion, can give rise to intergranular cracking whose progression can lead to leakage of primary fluid across the thickness of the tube.
In order to remedy this drawback, methods have.. been proposed for thermal or mechanical stress relaxation of the wall of the tubes of the bundle of a steam generator in the transition zone.
However, it is also necessary to have available repair methods which can be implemented on steam generators whose tube bundle has already suffered stress corrosion.
The method which seems most satisfactory for performing this repair consists in sheathing a part of the inner surface of the tube such that the sheath or sheathing sleeve covers the crack through the wall of the tube or which risks breaching this wall.
The sheathing sleeve, whose diameter is smaller than the internal diameter of the tube, is placed in the desired position inside this tube and is subjected to diametrical expansion by widening which guarantees both the mechanical strength and the seal of the fixing of the sleeve. Widening may be performed over the entire height of the sleeve or only in two zones of this sleeve corresponding to its upper and lower ends.
The sheathing sleeve may also be brazed inside the tube or fixed by a weld bead at each of its ends.
In certain cases, one end, preferably the upper end, of the sleeve is fixed by widening in the tube and the other end of the sleeve is fixed by welding.
Even if the tube is not fixed by crimping, it is necessary to ensure contact between the sheathing sleeve and the tube by using a widening operation in order to eliminate the radial play between the sheathing sleeve and the tube and to perform brazing or welding under satisfactory conditions.
Known sheathing methods effectively make it possible to repair tubes with defects resulting from cracks caused by stress corrosion and to avoid leakages of primary fluid into the secondary fluid. However, it has been observed that, after a certain operating time of the tubes repaired in this way, the tube bundle again had a certain level of leakage detected by monitoring the . radioactivity of the secondary water. On examination, it appeared that new defects had developed in the tubes, generally at the level of the upper end for fixing the sheath in the tube or in the immediate vicinity of this upper end.
The upper end of the sheathing sleeves which is located in the part of the tube which projects relative to the upper face of the tube plate and which is generally fixed by crimping inside the tube is located precisely in a zone where the tube is subject to a certain diametrical expansion and has a considerable concentration of stresses.
In the case of partial crimping of the tube, the transition zone is located above the crimped portion of the tube, inside the bore passing through the tube plate. Cracks usually appear in this transition zone. It is thus possible to envisage repairing the tube by crimping the tube itself, in the bore of the tube plate, above the transition zone.
However, there is a risk of new cracks subsequently appearing in the new transition zone created when complementary crimping of the tube is carried out.
A method described in FR-A-2,565,323 is known, which makes it possible to protect, against stress corrosion, a tube, such as a steam generator tube crimped into a tube plate and, in particular, the transition zone of this tube located in the vicinity of the exit face of the tube plate and corresponding to the separation zone between the expanded part of the tube inside the tube plate and the non-expanded part of the tube. This protection method consists in depositing, using electrolysis, a metallic layer on the inner surface of the tube after it has been fixed in the tube plate. The electrolytic coating makes it possible to insulate the inner surface of the tube, particularly in the zone where the wall of the tube has a high concentration of stresses, from the exchange fluid, such as the pressurized water circulated inside the tube.
However, a method of this type has never been used for repairing a tube by sheathing and involving. deformation by diametrical expansion of the tube in its part projecting relative to the tube plate or in the case of partial repair of a crimped tube by complementary crimping above the transition zone.